Compressor/expander switching methods and apparatus

ABSTRACT

The invention relates to compressors and expanders wherein the input signal has additively or subtractively combined therewith the output of a processing means which limits the further signal component contributed thereby so that, at small input levels, the input signal is substantially boosted or bucked whereas, at large input levels, the further signal component has negligible effect. The invention provides switching arrangements for switching efficiently between compressor and expander configurations, especially for use in or with tape recorders.

Elnite tates Patent Dolby [451 Apr. 24, 1973 [54] COMPRESSOR/EXPANDER Primary Examiner-Paul L. Gensler SWITCHING METHQDS AND Attorney-Robert F. OConnell APP AT AR Us [57] ABSTRACT [76] Inventor: Ray Milton Dolby, 346 Clapham A Road London England The invention relates to compressors and expanders wherein the input signal has additively or subtractively [22] Filed: Aug. 2, 1971 combined therewith the output of a processing means which limits the further signal component contributed thereby so that, at small input levels, the input signal is substantially boosted or bucked whereas, at large input levels, the further signal component has negligible effect. The invention provides switching arrangements for switching efficiently between compressor and expander configurations, especially for use in or with tape recorders.

45 Claims, 15 Drawing Figures [52] U.S. Cl ..333/14, 179/1 P, 179/100.2 K, 330/31, 330/35, 330/145, 330/151 [51] Int. Cl. ..H04b 1/64, H03g 7/00 [58] Field of Search ..333/14, 17; 328/152, 328/175; 330/151; 179/1 D, 1 P, 100.2 K

[56] References Cited UNITED STATES PATENTS 3,631,365 12/1971 Dolby ..333/14 7'0 METER IZJC 7104/ 22 I i WPUT F/LTfR f (FROM ulvr nv 0R FROM TAPE FURTHER PATH PLAYBACK) T 70 MON/70R i 55 33 T /V/(/I\ //;9(E)U/;EDUCT/ON v i re T I RECORD M4] AMP.

COMB/NE Patented April 24, 1973 '7 Sheets-Sheet 1 F/G, 7 F102 I FURTHER FURTHER PATH PATH "l r 70 [Z INPUT 'T 6 I 12 L77 'NPUT ourPuT F104 I3 FURTHER P I ATH FURTHfR l 7 l2 PATH a- INPUT OUTPUT INPUT H our/ 01 J 70 2 52 MON/TOR FURTHER PATH 524 r0 REC. FRO/7 m: iour/ ur L/A/f 53 Patented April 24, 1973 7 Sheets-Sheet 5 QQBI O2 w Wm kbni QWIHO MO WEOIKOQUZk $62 whim 4 \mowmmmtou ow 5:950 u mm.

vmmhmt Patented April 24, 1973 H08 P R MON/70R CIRCUIT T\ 24 70 35A i335 C t l REC. 25c.

| (ALI) 52A g I 26 I 25 FURTl/[R 518 PATH I To MON/TOR 55 o I 525 FROMREC. I LINE //v c I OUTPUTI R /NPU7' 4i 2% 51A FURTHER -26 PATH 255M0N/70R ?7'0 REC.

7 Sheets-Sheet 4 Patented April 24, 1973 7 Sheets-Sheet 6 FIG, A I I 1 FURTHER PATH *26 C 24 TO REC. I R 25 4 c 7 o P NON/TOR 1% 5 1 FROM R50 57 5 FIG. 73.

52- FURTHER 26 PATH 25 5o 8 P MON/TOR 5 HCROM REC. I To Ric LINE/NR 24A 51A R 0P 51B COMPRESSOR/EXPANDER SWITCHING METHODS AND APPARATUS RELATED APPLICATIONS 1. Ser. No. 173,261, filed Aug. 19, 1971 (a streamlined continuation of Ser. No. 880,481, filed Dec. 10, 1969, now abandoned, in turn a streamlined continuation of Ser. No. 569,615, filed Aug. 2, 1966, now abandoned), Ray M. Dolby.

2. Ser. No. 227,144, filed Feb. 17, 1972 (a continuation of Ser. No. 789,703, filed .Ian. 8, 1969, now abandoned, in turn a continuation-in-part of Ser. No. 569,615, filed Aug. 2, 1966, now abandoned), Ray M. Dolby.

3. Ser. No. 867,454, filed Oct. 20, 1969, now US. Pat. No. 3,631,365, issued Dec. 28, 1971, Ray M. Dolby.

BACKGROUND OF THE INVENTION Application ('1 referenced above discloses what are referred to as Type 1 compressors and expanders. Application (2) discloses Type 2 devices. For convenience FIGS. 1 to 4 herein illustrate the basic configurations as follows:

FIG. 1: Type 1 compressor.

FIG. 2: Type 1 expander.

FIG. 3: Type 2 compressor.

FIG. 4: Type 2 expander.

In each Figure a main signal path extends from an input terminal 10, through an additive combining means 11 to an output terminal 12. The second input to the additive combining means is provided by a socalled further path 13 the input of which is derived either from the input terminal (FIGS. 1 and 4) or from the output terminal (FIGS. 2 and 3). In the case of the compressors the further path component adds to or boosts the main path component. In the case of the expanders an inverting circuit 14 causes the further path component to subtract from or buck the main path component.

At low signal levels the further component bucks or boosts the main path component appreciably; the further path can include an amplifier. However, the further path has signal level limiting or restricting characteristics above a low-level threshold, such that, at high signal levels, the further path component has negligible effect on the main path component, e.g the further path component can be limited to a tenth or even less of the main path component at maximum input level.

The restricting action can be achieved in various ways, including narrowing the frequency band passed by the further path, as described in the referenced applications. In video applications various non-linear band limiting techniques can be employed. For audio applications the restricting action must be syllabic or linear, i.e. the action must occur with a time constant sufficiently long to avoid non-linear distortion of waveforms.

In relatively inexpensive cassette units and other home recorders economic considerations dictate that, as much as possible, the same circuits shall be employed both in recording and in playback since in such applications it is not essential for recording and playback to be performed simultaneously. Thus the same magnetic head and associated amplifiers can be utilised for the two functions, switching means being provided to establish appropriate operating conditions. There are other situations, both professional and nonprofessional, and including audio, video, and instrumentation applications, in which it is desirable to utilise as much as possible of the same heads and electronic circuitry for both the recording and playback functions.

It is an attribute of applicants noise reduction systems that the circuitry thereof can be transformed without difficulty from the record (encode) function to the complementary playback (decode) function. It is thus adaptable to the above-mentioned situations in which circuit economy is important.

For example, application (3) describes and shows Type 1 compressors and expanders particularly suited for use in conjunction with home audio tape recorders, one scheme of mode switching being described in the application.

The object of the present invention is to provide further, improved, and simplified switching methods, particularly for use in tape recorder applications.

THE INVENTION An important feature of the invention is that the circuit has connections for establishing both the compressor and expander configurations; in the simplest switching embodiments a single-pole, two-way switch is used to render the compressor and expander configurations selectively operative.

Another important result of the invention is to provide a monitor terminal such terminal being connected to the input of the further path in Type 1 devices. In Type 2 devices a monitoring signal is derived by subtractively combining the input to and the output from the further path. It will be shown that such connections make a signal available for monitoring during the recording mode and available as the output signal during playback, which signal is always the normal, noncomressed signal, without, however, requiring the use of any switching over and above that used for compression/ expansion mode changing.

Other features and advantages of the invention will appear from the following description and the claims.

DRAWINGS FIGS. 1 to 4 show the basic Type 1 and 2 configurations described above.

FIG. 5 is a schematic circuit diagram of a Type 1 switchable compressor/expander embodying the invention.

FIG. 6 shows the circuit of FIG. 5 in a complete tape deck layout.

FIG. 6A shows a modification of FIG. 6 using separate recording and playback heads.

FIG. 7 is a simplified schematic diagram of the layout of FIG. 6.

FIG. 8 is like FIG. 7 but shows a Type 2 layout.

FIG. 9 is like FIG. 7 but shows switching on the output side of a Type 1 layout.

FIG. 10 is like FIG. 9 but shows a Type 2 layout.

FIG. 11 shows an even simpler Type 1 layout.

FIG. 12 is like FIG. 11 but shows a Type 2 layout.

FIG. 13 shows an alternative way of effecting the switching in a Type 1 layout.

FIG. 14 is like FIG. 13 but shows a Type 2 layout.

DESCRIPTION OF EMBODIMENTS For simplicity, only a single channel is shown in each instance. In all Figures the letters R and P denote the positions of switches during recording and playback respectively (i.e. encoding and decoding).

FIG. 5 shows an input terminal which receives the input signal to be recorded (line-in) in the recording mode and the signal from the tape in the playback mode. The input signal is fed through a 19 kHz lowpass filter 21, for rejecting FM stereo multiplex signals and tape recorder bias signals, and an amplifier 22 to a terminal 23 which may be connected to a passive recording level meter circuit. For this purpose the amplifier is able to supply a high current from a low output impedance. The meter can also be connected to the point 35, but this connection requires a high output from amplifier 34. The theoretically optimum point for the meter is the monitor point 25, since the same signal is present at this point in both the recording and playback modes; however, this connection requires a high output capability from both amplifiers 22 and 34.

In the noise reduction system the meter has the main function of ensuring that the same level of signal is fed to the further path in both operating modes, so that the compressor/expander action will be truly complementary for correct decoding of the signal. The meter also has the function of ensuring that the signal levels in the noise reduction system are optimum with respect to system noise, overload point, and efficiency of noise reduction action. In recorders the meter can also have the further function of ensuring that the signal levels are optimum for the tape or other recording medium.

The output of the amplifier 22 is also applied to the R terminal of a single-pole two-way switch 24 whose common terminal C is connected to a monitor terminal 25, for driving a monitoring amplifier and loudspeaker, and also to the input of a further path 26.

The further path is basically of the form disclosed in application (3) and comprises a fixed filter 27, a variable cut-off filter 28, an amplifier 29 and limiting diodes 30 which suppress transients allowed through by the syllabic action of the further path 26. The output of the further path is added to the signal from the amplifier 22 by a combining circuit 31 comprising resistors 32 and 33 and an amplifier 34. The output of the amplifier 34 is connected to a recording amplifier terminal 35 and also to the P terminal of the switch 24. The further signal component can be switched off by means of switch 55; the compressor/expander then becomes a linear amplifier.

The further path is non-inverting, as represented by the sign whereas the amplifier 34 is inverting, as represented by the sign in the symbol therefor. Accordingly, in the recording mode, with the switch 24 as shown, the signal at the recording amplifier terminal 35 is the sum of the main path signal from amplifier 22 and the further path signal applied to resistor 33. The fact that the sum signal is inverted in polarity is immaterial to the recording amplifier. The signal available at the monitor terminal 25 is the unmodified input signal, whereas the signal at terminal 35 is the compressed signal.

In the playback mode the input signal at terminal 20 is obtained from the playback amplifier output. In this mode the input to the further path becomes the inverted output of the amplifier 34. Therefore the further path component subtracts from the main path component and expansion is effected. Therefore the normal, noncompressed (decoded) signal is established at the output of the amplifier 34 and, because the monitor terminal 25 is connected to the input to the further path, the normal, non-compressed signal also passes as output signal to the monitor.

The level restricting action of the further path 26 is achieved by shifting the cut-off frequency of the fiiter 28 higher by rendering an F ET 36 in the filter progressively more conductive. To this end a control signal for the PET is derived from any convenient point in the circuit; in the example shown the output of the further path 26 is amplified, rectified and smoothed by an amplifier 37, diode 38 and a smoothing circuit symbolised by a capacitor 39. The smoothed control signal is applied to the gate electrode of FET 36 to yield syllabic limiting action.

FIG. 6 shows the switched compressor/expander circuit of FIG. 5 included in a complete tape deck layout. The input terminal 20 is connected to the output of a pre-arnplifier/head-amplifier 40 which is connected to a microphone or other input signal terminal 14 by a switch 24A in the recording mode. The recording amplifier terminal 35 is connected through a recording amplifier 42 to one end 43A of the winding 43 of a recording/playback head 44. In the recording mode a switch 24B grounds the other end 43B of the winding 43. In the playback mode the switch 24A connects the end 433 of the head winding to the amplifier 40 and the switch 24B grounds the end 43A of the head winding.

The three switches 24, 24A and 24B are ganged together for recording/playback mode switching. Other ganged switches may be employed if required, e.g. to select the frequency response (equalisation) of the amplifier 40 appropriately and to connect a recording level control (and balance control in stereo applications) between the amplifier 40 and terminal 20 during recording, substituting a preset playback calibration controlduring playback.

The meter terminal 23 is connected to a meter circuit 45 and the monitor terminal 25 is connected to the output amplifier and loudspeaker (not shown). In recording mode the monitor signal is line-in; in playback it is the decoded tape signal.

FIG. 6A shows part of FIG. 6 modified by replacing the single head 44 by separate recording and playback heads 46 and 47 respectively. The winding of the head 47 is simply connected to the P terminal of switch 24A. The switch 24B is replaced by a switch 24C which connects the output of the recording amplifier 42 to the winding of the recording head 46 in recording mode I and to ground in playback mode.

The circuit of FIGS. 6 and 6A can alternatively be embodied in a separate noise reduction unit adapted to be used in conjunction with a conventional recorder. In this case the input terminal 20 is connected by the switch 24A to a microphone or other input signal terminal in the recording mode and to a terminal for receiving the recorder output in the playback mode. The terminals 25 and 35 are provided as above for connection to the recorder input terminal and the monitor facilities input terminal respectively.

The essential features of the layout of FIGS. 6 and 6A are summarised in FIG. 7. Since the same reference numerals are used again, detailed description of the circuit is not necessary. The circuit between terminal C on switch 24A and the output terminal 53 comprises the basic compressor/expander circuit. The rest of the connections show the input switching and monitor circuit necessary for most convenient and practical operation with tape recorders. The input signal terminal 41 is designated line-in. The terminal 50 INPUT FROM RECORDER corresponds to the point marked 50 in FIGS. 6 and 6A. The combining and inverting blocks 51 and 52 represent the combining and inverting functions respectively of the block 31 in FIG. 5. The recorder terminal 35 could also be connected to the output of block 52 instead of to the output of block 51. The compression or expansion action can be switched off by switch 55.

FIG. 8 corresponds to FIG. 7 but shows the modified configuration appropriate to Type 2 operation. The drawing to the right of input terminal C on switch 24A and to the left of the dotted line comprises the basic compressor/expander circuit. The rest of the circuit shows the input switching and monitor circuitry necessary for most convenient and practical operation with tape recorders.

In contrast with Type 1 compressor/expanders, in which a monitoring signal is available at the input to the further path, there is no single point in Type 2 circuits at which a normal, noncompressed signal is available in both the recording and playback modes. In Type 2 circuits a further switch, ganged with S24 and 824A, can be incorporated to connect the monitor terminal 25 to the C terminal of switch 24A (i.e. the input) in the recording mode and to the output of the combining means 51A (expansion output) in the playback mode. However, for applications in which it is desirable to minimise the amount of switching used, it is possible in Type 2 devices to derive a monitor signal by subtractively combining the input to and the output from the further path. In the recording mode the input to the further path is the input signal component plus the further signal; in the monitoring scheme the further signal component is cancelled at the monitoring point, leaving only the main input signal component. In the playback mode the input to the further path is the main signal component only. The signal at the monitor point will therefore include the further signal subtractively combined i.e. Type 2 expansion. The extra subtractive combining means required is represented in FIG. 8 by the inverter 52B and additive combining means 518. The signal to the tape recorder (terminal 35) in recording mode may be obtained at the input to the further path or alternatively at the output of the combining means 51A.

An advantage of FIGS. 7 and 8 is that switching between compressor and expander configurations is effected at the input side of the further path 26 where conditions are typically low impedance. Accordingly no stray signal pickup is likely at the disconnected terminal of the switch 24. It is nevertheless possible to effeet the switching at the output side of the further path as shown in FIGS. 9 and 10 for Type 1 and Type 2 operation respectively. Again in FIG. 9 (Type 1) the monitor signal at terminal 25 is always the normal, noncompressed signal. In FIG. 10 the monitoring signal, if required, is derived in the same manner as in FIG. 8,

i.e. by inverter 52B and adder 51C, which subtractively combine the input to and the output from the further path.

Care is necessary in the detailed design of the circuits of FIGS. 9 and 10 since the conditions at the points R and P of switch 24 are typically high impedance, and stray pick-up can occur at whichever terminal of the switch 24 is disconnected. The problem can be overcome, at the expense of further switching complexity, by switching the unused terminal of the switch 24 to ground. An added switch 24D for performing this function is shown in FIG. 9 only.

It is possible to eliminate the switch 24A as shown for the case of input side switching in FIG. 11 (Type 1) and FIG. 12 (Type 2). Separate filters for eliminating the bias and multiplex tones are necessary in the connections from terminals 50 and 41 respectively. (In FIGS. 7 and 10 a single filter can follow the switch 24A as shown in FIGS. 5 and 6). If, however, it is certain that an input (e.g. from a microphone) and a replay signal from the recorder will never occur together, the line-in and replay signals (to 41 and 50) can be combined together and passed through a single filter whose output is connected to both the terminals 41 and 50 in FIGS. 11 and 12. In FIG. 12, terminal 35 can alternatively be connected to the input of the further path.

As so far described, the switch 24 completes the required connection and breaks the unwanted connection. In another embodiment both connections can be permanently present and the switch used to ground the unwanted connection. This is illustrated in FIG. 13 (Type 1) and FIG. 14 (Type 2). It will be noted that the switching is effected on the output side of the further path. In fact, FIG. 13 is basically the same as FIG. 9, omitting the switch 24 of FIG. 9 and utilising the switch 24 in FIG. 13 in a manner similar to that of the switch 24D of FIG. 9, isolating means being provided so that the switch does not short out the further signal inputs of both combining means. Such an isolating means, which could simply be a resistor, is represented by block 54 in FIG. 14.

What is claimed is:

l. A selectively switchable signal compressor/expander comprising a main path extending between an input terminal and a first output terminal and including means for combining a further signal with the signal in said main path, a further path having an input and output for providing said further signal, first connecting means operative to establish a first connection in which said further path input is connected to a point in said main path and said further path output is connected to said combining means for establishing a compressor configuration whereby said further signal boosts said signal in said main path, second connecting means operative to establish a second connection in which said further path input is connected to a point in said main path and said further path output is connected to said combining means for establishing an expander configuration whereby said further signal bucks said signal in said main path, said further path having signal level restricting characteristics above a low-level threshold, such that said boosting and bucking are significant only at low input signal levels,

switching means for rendering said first connecting means effective and said second connecting means ineffective in a compressor mode and for rendering said first connecting means ineffective and said second connecting means effective in an expander mode,

and a terminal connected to said input of said further path.

2. A compressor/expander according to claim 1, in which the signals at said further path input and output are subtractively combined and fed to an output terminal.

3. A compressor/expander according to claim 1, in which switch means are provided for rendering said further signal ineffective, whereby signals are transferred from said input terminal to one or more output terminals in a substantially proportional manner.

4. A compressor/expander according to claim 1, wherein said first connecting means connect said further path input to a first point in said main path preceding said combining means and said second connecting means connect said further path input to a second point in said main path following said combining means.

5. A compressor/expander according to claim 4, wherein the signal path polarities are arranged such that the signals in said combining means combine additively when said first connecting means is effective and subtractively when said second connecting means is effective.

6. A compressor/expander according to claim 4, wherein the connection of said further path output to said combining means is by way of a permanent connection effective in both said first and second connections, and said switching means comprise a single-pole, two-way switch for connecting said further path input selectively to said first or second point.

7. A compressor/expander according to claim 6, comprising a further single-pole, two-way switch for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said single-pole switches being ganged.

8. A compressor/expander according to claim 1, wherein said combining means comprise first combining means followed by second combining means, said first connecting means connect said further path output to one of said first and second combining means, said second connecting means connect said further path output to the other of said first and second combining means, and both said connecting means connect said further path input to a point between said first and second combining means.

9. A compressor/expander according to claim 8, wherein the connection of said further path input to said point between said first and second combining means is by way ofa permanent connection effective in both said first and second connections.

10. A compressor/expander according to claim 9, wherein said switching means comprises a single-pole, two-way switch for connecting said further path output selectively to said first or said second combining means.

11. A compressor/expander according to claim 10, wherein said switching means comprise a second single-pole, twoway switch ganged to the switch recited in claim 10 for grounding the connection to the nonselected combining means.

12. A compressor/expander according to claim 10, comprising a further single-pole, two-way switch for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said single-pole switches being ganged.

13. A compressor/expander according to claim 9, wherein said connection means include first and second permanent connections from said further path output to said first and second combining means respectively, and said switching means comprise a single-pole, two-way switch for selectively clamping at least one of said permanent connections to a reference potential.

14. A compressor/expander according to claim 13 comprising a further single-pole, two-way switch for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said single-pole switches being ganged.

15. A compressor/expander according to claim 8, wherein said connecting means connect said further path output permanently to said first and second combining means, wherein said main path is interrupted between said first and second combining means wherein one input of said second combining means is connected to said further path input, wherein a second input terminal is provided, and wherein said switching means comprise a single-pole, two-way switch for selectively connecting said further path input to the output of said first combining means and to said second input terminal.

16. A selectively switchable signal compressor/expander comprising a main path extending between an input terminal and a first output terminal and including means for combining a further signal with the signal in said main path, a further path having an input and output for providing said further signal connecting means connecting said further path output to said combining means, and switching means for selectively connecting said further path input to one of said input and output terminals for establishing a compressor configuration whereby said further signal boosts said signal in said main path, or for connecting said further path input to the other of said input and output terminals for establishing an expander configuration whereby said further signal bucks said signal in said main path, said further path having signal level restricting characteristics above a low-level threshold,- such that said boosting and bucking are significant only at low input signal levels.

17. A compressor/expander according to claim 16, wherein the signal path polarities are arranged such that the signals in said combining means combine additively when said input is selectively connected to said input terminal and subtractively when said input is selectively connected to said output terminal.

18. A compressor/expander according to claim 17, further comprising a second output terminal connected to said input of said further path.

19. A compressor/expander according to claim 16, wherein the signal path polarities are arranged such that the signals in said combining means combine additively when said input is selectively connected to said output terminal and subtractively when said input is selectively connected to said input terminal.

20. A compressor/expander according to claim 19, comprising a further output terminal which is connected to the output of a combining means which subtractively combines the signals at said further path input and said further path output.

21. A compressor/expander according to claim 16, comprising a further switching means for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said switching means being ganged.

22. A selectively switchable signal compressor/expander comprising a main path extending between an input terminal and a first output terminal and including a first followed by a second combining means for combining a further signal with the signal in said main path, a further path having an input and an output for providing said further signal, connecting means connecting said further path input to a point in said main path between said combining means, switching means for selectively connecting said further path output to one of said combining means for establishing a compressor configuration whereby said further signal boosts said signal in said main path or for connecting said further path output to the other of said combining means for establishing an expander configuration whereby said further signal bucks said signal in said main path, said further path having signal level restricting characteristics above a low-level threshold, such that said boosting and bucking are significant only at low input signal levels, and a second output terminal connected to said input of said further path.

23. A compressor/expander according to claim 22, wherein the signal path polarities are arranged such that the signals in said first combining means combine subtractively and in the second additively.

24. A compressor/expander according to claim 22, wherein the signal path polarities are arranged such that the signals in said first combining means combine additively and in the second subtractively.

25. A signal compressor/expander according to claim 24, wherein the further signal input of said second combining means is connected to the output of said further path.

26. A compressor/expander according to claim 22, comprising further switching means for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said switching means being ganged.

27. A selectively switchable signal compressor/expander comprising first and second input terminals, first and second output terminals, first combining means having a first input connected to said first input terminal, a further path having an input and output, first connecting means connecting said further path output to the second input of said first combining means for establishing an expander configuration whereby the signal path polarities are so arranged that the signals in said first combining means combine subtractively, second combining means with a first input, second connecting means connecting said further path output to the second input of said second combining means for establishing a compressor configuration whereby the signals in said second combining means combine subtractively, said further path having signal level restricting characteristics above a low-level iii threshold, such that the boosting and bucking actions produced are significant only at low input signal levels,

and switching means for connecting said further path input selectively to said second input terminal or to the output of said first combining means, said first and second output terminals being connected respectively to said further path input and the output of said second combining means,and said first input of said second combining means being connected to a point which, at least in compression mode, is the same as said input.

28. A selectively switchable signal compressor/expander comprising first and second input terminals, first and second output terminals, first combining means having a first input connected to said first input terminal, a further path having an input and output, first connecting means connecting said further path output to the second input of said first combining means for establishing a compressor configuration whereby the signal path polarities are so arranged that the signals in said first combining means combine additively, second combining means with a first input connected to said input, second connecting means connecting said further path output to the second input of said second combining means for establishing an expander configuration whereby the signal path polarities are so arranged that the signals in said second combining means combine subtractively, said further path having signal level restricting characteristics above a lowlevel threshold such that the boosting and bucking actions produced are significant only at low input signal levels,

and switching means for connecting said further path input terminal and a first output terminal and includinga first followed by a second means for combining a further signal with the signal in said main path, a further path having an input and an output for providing said further signal, first connecting means connecting said input to a point in said main path between said combining means, second connecting means connecting said output to one of said combining means for establishing a compressor configuration whereby said further signal boosts said signal in said main path, third means connecting said output to the other of said combining means for establishing an expander configuration whereby said further signal bucks said signal in said main path, said further path having signal level restricting characteristics, above a low-level threshold such that said boosting and bucking are significant only at low input signal levels, and

switching means for selectively grounding at least one of said second or said third connecting means. 30. A compressor/expander according to claim 29, further comprising a second output terminal connected to said input of said further path.

31. A compressor/expander according to claim 29, wherein the signal path polarities are arranged such that the signals in said first combining means combine subtractively and in the second additively.

32. A compressor/expander according to claim 29, wherein the signal path polarities are arranged such that the signals in said first combining means combine additively and in the second subtractively.

33. A circuit for use with a tape recorder comprising a selectively switchable signal compressor-expander comprising a first terminal for receiving an input signal, a second terminal for receiving a signal played back from the recorder, a third terminal, first switching means for selectively connecting said third terminal to said first terminal or said second terminal, a fourth terminal, combining means connected between said third and fourth terminals, a further path having an input and an output, a fifth terminal connected to said input, a recording amplifier connected to one of said fourth terminal and a point which, at least in compression mode, is the same as said fifth terminal, the other of said fourth and fifth terminals providing a monitoring terminal, first connecting means operative to establish a first connection in which said further path input is connected to a point between said third and fourth terminals and said further path output is connected to said combining means for establishing a compressor configuration whereby the signal path polarities are so arranged that the signal from said further path boosts the signal at one of said fourth and fifth terminals, second connecting means operative to establish a second connection in which said further path input is connected to a point between said third and fourth terminals and said further path output is connected to said combining means for establishing an expander configuration whereby the signal path polarities are so arranged that the signal from said further path bucks the signal at one of said fourth and fifth terminals, said further path having signal level restricting characteristics above a low level threshold, such that said boosting and bucking are significant only at low input signal levels, and

second switching means ganged to said first switching means for rendering said first connecting means effective and said second connecting means ineffective in a compressor mode and for rendering said first connecting means ineffective and said second connecting means effective in an expander mode.

34 A circuit according to claim 33, wherein said first connecting means connect said further path input to a first point in said main path preceding said combining means and said second connecting means connect said further path input to a second point in said main path following said combining means.

35. A circuit according to claim 34, wherein said first connecting means connect said further path input to a first point in said main path following said combining means and said second connecting means connect said further path input to a second point in said main path preceding said combining means.

36. A circuit according to claim 35, comprising a monitoring output terminal which is connected to the output of a combining means which subtractively combines the signals at said further path input and output.

37. A circuit according to claim 34, wherein said second switching means selectively connect said further path input to said first or said second point.

38. A circuit according to claim 35, wherein said second switching means selectively connect said further path input to said first or said second point.

353. A circuit according to claim 33, wherein said combining means comprise first combining means followed by second combining means, said first connecting means connect said further path output to one of said first and second combining means, said second connecting means connect said further path output to the other of said first and second combining means, and both said connecting means connect said input to a point between said first and second combining means.

40. A circuit according to claim 39, comprising a monitoring output terminal which is connected to the output of a combining means which subtractively combines the signals at said further path input and output.

41. A circuit according to claim 39, wherein the connection of said further path input to said point between said first and second combining means is by way of a permanent connection effective in both said first and second connections.

42. A circuit according to claim 41, wherein said switching means comprise a single-pole, two-way switch for connecting said further path output selectively to said first or said second combining means.

43. A circuit according to claim 42, wherein said switching means comprise a second singlepole, twoway switch ganged to the switch recited in claim 42 for grounding the further signal connection to at least the said first combining means.

44. A circuit according to claim 41, wherein said connecting means include first and second permanent connections from said further path output to said first and second combining means respectively, and said switching means comprise a single-pole, two-way switch for selectively clamping at least said first permanent connection to a reference potential.

45. A circuit for use with a tape recorder comprising a selectively switchable signal compressor/expander comprising a first terminal for receiving an input signal, a second terminal for receiving a signal played back from the recorder, first and second combining means, a

third terminal connected to the output of said second combining means, a further path having an input and an output, an input of said second combining means connected to said further path input, a recording amplifier connected to one of said third terminal and a fourth terminal, the other of said third and fourth terminals providing a monitoring output terminal, first connecting means connecting said further path output to one of said combining means for establishing a compressor configuration whereby the signal path polarities are so arranged that the signal from said further path boosts the signal at one of said third and fourth terminals, second connecting means connecting said further path output to the other of said combining means for establishing an expander configuration whereby the signal path polarities are so arranged that the signal from said further path bucks the signal at one of said third and fourth terminals, said further path having signal level restricting characteristics above a low-level threshold, such that said boosting and bucking are significant only at low input signal levels, switching means selectively connecting said further path input to one of said first and second input terminals or the output of said first combining means, the other of said first and second terminals being connected to an input of said first combining means, and said fourth terminal being connected to said further path input at least in one setting of said switching means. 

1. A selectively switchable signal compressor/expander comprising a main path extending between an input terminal and a first output terminal and including means for combining a further signal with the signal in said main path, a further path having an input and output for Providing said further signal, first connecting means operative to establish a first connection in which said further path input is connected to a point in said main path and said further path output is connected to said combining means for establishing a compressor configuration whereby said further signal boosts said signal in said main path, second connecting means operative to establish a second connection in which said further path input is connected to a point in said main path and said further path output is connected to said combining means for establishing an expander configuration whereby said further signal bucks said signal in said main path, said further path having signal level restricting characteristics above a low-level threshold, such that said boosting and bucking are significant only at low input signal levels, switching means for rendering said first connecting means effective and said second connecting means ineffective in a compressor mode and for rendering said first connecting means ineffective and said second connecting means effective in an expander mode, and a terminal connected to said input of said further path.
 2. A compressor/expander according to claim 1, in which the signals at said further path input and output are subtractively combined and fed to an output terminal.
 3. A compressor/expander according to claim 1, in which switch means are provided for rendering said further signal ineffective, whereby signals are transferred from said input terminal to one or more output terminals in a substantially proportional manner.
 4. A compressor/expander according to claim 1, wherein said first connecting means connect said further path input to a first point in said main path preceding said combining means and said second connecting means connect said further path input to a second point in said main path following said combining means.
 5. A compressor/expander according to claim 4, wherein the signal path polarities are arranged such that the signals in said combining means combine additively when said first connecting means is effective and subtractively when said second connecting means is effective.
 6. A compressor/expander according to claim 4, wherein the connection of said further path output to said combining means is by way of a permanent connection effective in both said first and second connections, and said switching means comprise a single-pole, two-way switch for connecting said further path input selectively to said first or second point.
 7. A compressor/expander according to claim 6, comprising a further single-pole, two-way switch for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said single-pole switches being ganged.
 8. A compressor/expander according to claim 1, wherein said combining means comprise first combining means followed by second combining means, said first connecting means connect said further path output to one of said first and second combining means, said second connecting means connect said further path output to the other of said first and second combining means, and both said connecting means connect said further path input to a point between said first and second combining means.
 9. A compressor/expander according to claim 8, wherein the connection of said further path input to said point between said first and second combining means is by way of a permanent connection effective in both said first and second connections.
 10. A compressor/expander according to claim 9, wherein said switching means comprises a single-pole, two-way switch for connecting said further path output selectively to said first or said second combining means.
 11. A compressor/expander according to claim 10, wherein said switching means comprise a second single-pole, two-way switch ganged to the switch recited in claim 10 for grounding the connection to the non-selected combining means.
 12. A compressor/expAnder according to claim 10, comprising a further single-pole, two-way switch for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said single-pole switches being ganged.
 13. A compressor/expander according to claim 9, wherein said connection means include first and second permanent connections from said further path output to said first and second combining means respectively, and said switching means comprise a single-pole, two-way switch for selectively clamping at least one of said permanent connections to a reference potential.
 14. A compressor/expander according to claim 13 comprising a further single-pole, two-way switch for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said single-pole switches being ganged.
 15. A compressor/expander according to claim 8, wherein said connecting means connect said further path output permanently to said first and second combining means, wherein said main path is interrupted between said first and second combining means wherein one input of said second combining means is connected to said further path input, wherein a second input terminal is provided, and wherein said switching means comprise a single-pole, two-way switch for selectively connecting said further path input to the output of said first combining means and to said second input terminal.
 16. A selectively switchable signal compressor/expander comprising a main path extending between an input terminal and a first output terminal and including means for combining a further signal with the signal in said main path, a further path having an input and output for providing said further signal connecting means connecting said further path output to said combining means, and switching means for selectively connecting said further path input to one of said input and output terminals for establishing a compressor configuration whereby said further signal boosts said signal in said main path, or for connecting said further path input to the other of said input and output terminals for establishing an expander configuration whereby said further signal bucks said signal in said main path, said further path having signal level restricting characteristics above a low-level threshold, such that said boosting and bucking are significant only at low input signal levels.
 17. A compressor/expander according to claim 16, wherein the signal path polarities are arranged such that the signals in said combining means combine additively when said input is selectively connected to said input terminal and subtractively when said input is selectively connected to said output terminal.
 18. A compressor/expander according to claim 17, further comprising a second output terminal connected to said input of said further path.
 19. A compressor/expander according to claim 16, wherein the signal path polarities are arranged such that the signals in said combining means combine additively when said input is selectively connected to said output terminal and subtractively when said input is selectively connected to said input terminal.
 20. A compressor/expander according to claim 19, comprising a further output terminal which is connected to the output of a combining means which subtractively combines the signals at said further path input and said further path output.
 21. A compressor/expander according to claim 16, comprising a further switching means for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said switching means being ganged.
 22. A selectively switchable signal compressor/expander comprising a main path extending between an input terminal and a first output terminal and including a first followed by a second combining means for combining a further signal with the signal in said main path, A further path having an input and an output for providing said further signal, connecting means connecting said further path input to a point in said main path between said combining means, switching means for selectively connecting said further path output to one of said combining means for establishing a compressor configuration whereby said further signal boosts said signal in said main path or for connecting said further path output to the other of said combining means for establishing an expander configuration whereby said further signal bucks said signal in said main path, said further path having signal level restricting characteristics above a low-level threshold, such that said boosting and bucking are significant only at low input signal levels, and a second output terminal connected to said input of said further path.
 23. A compressor/expander according to claim 22, wherein the signal path polarities are arranged such that the signals in said first combining means combine subtractively and in the second additively.
 24. A compressor/expander according to claim 22, wherein the signal path polarities are arranged such that the signals in said first combining means combine additively and in the second subtractively.
 25. A signal compressor/expander according to claim 24, wherein the further signal input of said second combining means is connected to the output of said further path.
 26. A compressor/expander according to claim 22, comprising further switching means for connecting said input terminal selectively to a terminal for receiving a signal to be recorded or to a terminal for receiving a playback signal, said switching means being ganged.
 27. A selectively switchable signal compressor/expander comprising first and second input terminals, first and second output terminals, first combining means having a first input connected to said first input terminal, a further path having an input and output, first connecting means connecting said further path output to the second input of said first combining means for establishing an expander configuration whereby the signal path polarities are so arranged that the signals in said first combining means combine subtractively, second combining means with a first input, second connecting means connecting said further path output to the second input of said second combining means for establishing a compressor configuration whereby the signals in said second combining means combine subtractively, said further path having signal level restricting characteristics above a low-level threshold, such that the boosting and bucking actions produced are significant only at low input signal levels, and switching means for connecting said further path input selectively to said second input terminal or to the output of said first combining means, said first and second output terminals being connected respectively to said further path input and the output of said second combining means, and said first input of said second combining means being connected to a point which, at least in compression mode, is the same as said input.
 28. A selectively switchable signal compressor/expander comprising first and second input terminals, first and second output terminals, first combining means having a first input connected to said first input terminal, a further path having an input and output, first connecting means connecting said further path output to the second input of said first combining means for establishing a compressor configuration whereby the signal path polarities are so arranged that the signals in said first combining means combine additively, second combining means with a first input connected to said input, second connecting means connecting said further path output to the second input of said second combining means for establishing an expander configuration whereby the signal path polarities are so arranged that the signals in said second combining means combine subtractively, said further path having signal Level restricting characteristics above a low-level threshold such that the boosting and bucking actions produced are significant only at low input signal levels, and switching means for connecting said further path input selectively to said second input terminal or to the output of said first combining means, said second output terminal being connected to the output of said second combining means, and said first output terminal being connected to a point which, at least in compression mode, is the same as said input.
 29. A selectively switchable signal compressor/expander comprising a main path extending between an input terminal and a first output terminal and including a first followed by a second means for combining a further signal with the signal in said main path, a further path having an input and an output for providing said further signal, first connecting means connecting said input to a point in said main path between said combining means, second connecting means connecting said output to one of said combining means for establishing a compressor configuration whereby said further signal boosts said signal in said main path, third means connecting said output to the other of said combining means for establishing an expander configuration whereby said further signal bucks said signal in said main path, said further path having signal level restricting characteristics, above a low-level threshold such that said boosting and bucking are significant only at low input signal levels, and switching means for selectively grounding at least one of said second or said third connecting means.
 30. A compressor/expander according to claim 29, further comprising a second output terminal connected to said input of said further path.
 31. A compressor/expander according to claim 29, wherein the signal path polarities are arranged such that the signals in said first combining means combine subtractively and in the second additively.
 32. A compressor/expander according to claim 29, wherein the signal path polarities are arranged such that the signals in said first combining means combine additively and in the second subtractively.
 33. A circuit for use with a tape recorder comprising a selectively switchable signal compressor-expander comprising a first terminal for receiving an input signal, a second terminal for receiving a signal played back from the recorder, a third terminal, first switching means for selectively connecting said third terminal to said first terminal or said second terminal, a fourth terminal, combining means connected between said third and fourth terminals, a further path having an input and an output, a fifth terminal connected to said input, a recording amplifier connected to one of said fourth terminal and a point which, at least in compression mode, is the same as said fifth terminal, the other of said fourth and fifth terminals providing a monitoring terminal, first connecting means operative to establish a first connection in which said further path input is connected to a point between said third and fourth terminals and said further path output is connected to said combining means for establishing a compressor configuration whereby the signal path polarities are so arranged that the signal from said further path boosts the signal at one of said fourth and fifth terminals, second connecting means operative to establish a second connection in which said further path input is connected to a point between said third and fourth terminals and said further path output is connected to said combining means for establishing an expander configuration whereby the signal path polarities are so arranged that the signal from said further path bucks the signal at one of said fourth and fifth terminals, said further path having signal level restricting characteristics above a low-level threshold, such that said boosting and bucking are significant only at low input signal levels, and second switching means ganged to said first switching means for renderIng said first connecting means effective and said second connecting means ineffective in a compressor mode and for rendering said first connecting means ineffective and said second connecting means effective in an expander mode.
 34. A circuit according to claim 33, wherein said first connecting means connect said further path input to a first point in said main path preceding said combining means and said second connecting means connect said further path input to a second point in said main path following said combining means.
 35. A circuit according to claim 34, wherein said first connecting means connect said further path input to a first point in said main path following said combining means and said second connecting means connect said further path input to a second point in said main path preceding said combining means.
 36. A circuit according to claim 35, comprising a monitoring output terminal which is connected to the output of a combining means which subtractively combines the signals at said further path input and output.
 37. A circuit according to claim 34, wherein said second switching means selectively connect said further path input to said first or said second point.
 38. A circuit according to claim 35, wherein said second switching means selectively connect said further path input to said first or said second point.
 39. A circuit according to claim 33, wherein said combining means comprise first combining means followed by second combining means, said first connecting means connect said further path output to one of said first and second combining means, said second connecting means connect said further path output to the other of said first and second combining means, and both said connecting means connect said input to a point between said first and second combining means.
 40. A circuit according to claim 39, comprising a monitoring output terminal which is connected to the output of a combining means which subtractively combines the signals at said further path input and output.
 41. A circuit according to claim 39, wherein the connection of said further path input to said point between said first and second combining means is by way of a permanent connection effective in both said first and second connections.
 42. A circuit according to claim 41, wherein said switching means comprise a single-pole, two-way switch for connecting said further path output selectively to said first or said second combining means.
 43. A circuit according to claim 42, wherein said switching means comprise a second single-pole, two-way switch ganged to the switch recited in claim 42 for grounding the further signal connection to at least the said first combining means.
 44. A circuit according to claim 41, wherein said connecting means include first and second permanent connections from said further path output to said first and second combining means respectively, and said switching means comprise a single-pole, two-way switch for selectively clamping at least said first permanent connection to a reference potential.
 45. A circuit for use with a tape recorder comprising a selectively switchable signal compressor/expander comprising a first terminal for receiving an input signal, a second terminal for receiving a signal played back from the recorder, first and second combining means, a third terminal connected to the output of said second combining means, a further path having an input and an output, an input of said second combining means connected to said further path input, a recording amplifier connected to one of said third terminal and a fourth terminal, the other of said third and fourth terminals providing a monitoring output terminal, first connecting means connecting said further path output to one of said combining means for establishing a compressor configuration whereby the signal path polarities are so arranged that the signal from said further path boosts the signal at one of said third and fourth terminals, second connecting means connEcting said further path output to the other of said combining means for establishing an expander configuration whereby the signal path polarities are so arranged that the signal from said further path bucks the signal at one of said third and fourth terminals, said further path having signal level restricting characteristics above a low-level threshold, such that said boosting and bucking are significant only at low input signal levels, switching means selectively connecting said further path input to one of said first and second input terminals or the output of said first combining means, the other of said first and second terminals being connected to an input of said first combining means, and said fourth terminal being connected to said further path input at least in one setting of said switching means. 